1. Field of the Invention
The present patent application claims priority under the Paris Convention based on Japanese Patent Application No. 2008-25674 (filed on Feb. 5, 2008), and the entire content of the aforementioned application is herein incorporated by reference.
The present invention relates to an energy ray-curable ink composition, particularly to an energy ray-curable ink composition to be used for an ink-jet recording system.
2. Description of the Related Art
As an ink composition to be applied for an ink-jet recording system, for example, an aqueous ink containing water as a main component and an oil-based ink containing an organic solvent as a main component have conventionally been used. However, intense interest has been shown towards a solvent-free type energy ray-curable ink composition in which an ink is cured by irradiating with energy ray (for example, ultraviolet rays) so as to suppress blur of the printed material (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-214279).
As means for curing the energy ray-curable ink composition described above, for example, a low-pressure mercury lamp, a high-pressure mercury lamp, an ultrahigh-pressure mercury lamp, a xenon lamp and a metal halide lamp have been used. However, the energy ray-curable ink composition has a problem such as poor adhesion since a polymerizable compound usually has low reactivity and sufficient curability is not obtained due to oxygen inhibition when energy ray having high energy is not irradiated. Therefore, excess irradiation is performed so as to improve curability and an energy ray curing treatment is used in combination with a heat treatment so as to improve adhesion to a recording medium. However, when the irradiation means described above is used, the size of the entire mage recording device increases and also power consumption increases. Since heat is likely to be generated when irradiated with ultraviolet rays using a mercury lamp or a metal halide lamp, there arises a problem that, when a thin film is used as the recording medium, curl and waviness of the thin film are caused by excess irradiation with energy ray and a heat treatment. Therefore, it is required to develop an ink composition which has high curing sensitivity to energy ray and good adhesion to the recording medium.
Thus, the present inventors have previously proposed an ink composition containing, as a polymerizable compound, a polyfunctional monomer having excellent curing sensitivity, and an ink composition containing the polyfunctional monomer and also containing, as a photopolymerization initiator, an α-aminoalkylphenone-based compound capable of easily generating a radical so as to provide an ink composition which is excellent in curability and adhesion even when irradiated with energy ray having low energy (see Japanese Unexamined Patent Publication (Kokai) No. 2006-131883 and Japanese Unexamined Patent Publication (Kokai) No. 2006-131884).
However, the ink composition capable of curing with energy ray having low energy easily causes initiation of polymerization with heat or light during storage since a monomer having excellent curing sensitivity and a photopolymerization initiator must be used, leading to deterioration of storage stability. Although the addition of an antigelling agent is proposed so as to improve storage stability, the addition causes a problem such as inhibition of the polymerization reaction when irradiated with energy ray.